Aegis was created in response to these problems. Aegis is preflexed at 45° and anatomically tapered. This simple yet very significant departure from the current straight, windsock shaped liner designs provides the following benefits:
- Comfortable sitting at 90° knee flexion for extended periods
- Minimized wrinkling in the popliteal region
- Safe and far lower internal liner pressures in all ranges of motion
- Reduced patella discomfort and pressure
- Increased range of motion at the knee
- Absence of knee extension moment when knee is flexed
Negative compliance was tested at -10, -10, and -30 pounds of pull. It was calculated that normal prosthetic usage can develop between 10 to 20 pounds of pull during swing phase. These texts indicated that all three liners performed similarly regarding negative and positive compliance. It appears that a fabric matrix does not reduce compliance at all, although it is a advertised to do just that. The data confirmed our initial hypothesis that compliance occurs as a consequence of tissue mobility and cannot be reduced by a single layer of fabric.
Tests also indicate that the addition of a fabric matrix increases liner stiffness and subsequently reduces liner compliance (adhesion) to the limb. This can result in a loss of liner to tissue contact in various ranges of motion. Loss of skin adhesion to the liner can cause air pockets, friction, blistering, and skin irritation and breakdown.
The only effective means to reduce negative compliance (pistoning) is to roughen the inner surface of the socket or provide a proximal auxiliary suspension. As a consequence of these preliminary tests we made the following conclusions:
- A fabric matrix is irrelevant
- A fabric matrix reduces skin to liner contact
- Roughening the inner socket surface reduces both negative and positive compliance
The development of a silicone liner that is preflexed and tapered required considerable engineering time and team expertise. Residual limb length, which until now was considered unimportant, is a required measurement when ordering an Aegis silicone liner. Liner lengths of 4,6,8 and 10 inches are available in all circumferential sizes. Limb length is determined by measuring from the midpatella tendon to the distal end.
Circumferential sizes range from 12 through 30 cm (even numbers). Presently, only liner sizes 18 cm through 26 cm are available. The remaining sizes will become available in the next few months. The sizing chart (above) anticipates the appropriately sized liner required for proper limb compression and fit. Aegis liners are available with or without a shuttle umbrella.
The Aegis transtibial prefabricated silicone liner is unique in design and without peer. Aegis responds to the concerns of prosthetists and the needs of their patients with its innovative design and anatomical considerations.